Radio frequency (RF) filters are key components in any wireless system and as these systems continue to be miniaturized, the pressure on filter technology to shrink as well without compromising performance continues. Handheld systems and their associated volumes have generated strong interest in filter technologies that show promise for lower cost and smaller size. Bulk acoustic wave resonators may be built in an integrated circuit using standard integrated circuit manufacturing techniques, thereby offering low cost.
Typical bulk acoustic wave (BAW) resonators currently in production consist of a piezoelectric material sandwiched between a lower metallic electrode and an upper metallic electrode. When an alternating electric field is placed across the piezoelectric material by way of the electrodes, the piezoelectric structure mechanically deforms in a periodic manner and generates a standing acoustic wave.
The frequency of the acoustic wave depends upon the thickness of the piezoelectric material and the thickness or mass of the lower and upper metallic electrodes. Variation in the thickness of these materials across a semiconductor wafer results in variation in the frequency of the BAW resonator. When the specified range of the BAW resonator is narrow, significant yield loss may result from across wafer thickness non-uniformity of the piezoelectric material and the metallic electrode layers.
A typical BAW resonator is depicted in FIG. 1. The BAW resonator 110 consists of a piezoelectric material 104 sandwiched between a lower metal electrode 102 and an upper metal electrode 106. The BAW depicted in FIG. 1 is a solidly mounted resonator formed directly on a substrate 100 which may be an integrated circuit. Several layers of alternating low-acoustic and high-acoustic material form an underlying Bragg acoustic reflector 108. The layers in the Bragg acoustic reflector have a thickness corresponding to one-quarter of the wavelength of the fundamental resonant frequency of the BAW resonator 110 and reflect the resonant acoustic wave. A second Bragg acoustic reflector 112 may be formed over the BAW to additionally enhance the fundamental resonance frequency of the BAW resonator. Dielectric protective overcoat (PO) 114 is formed over the BAW and electrical contacts 116 and 118 are formed through openings in the PO to the bottom BAW resonator electrode 102 and top BAW resonator electrode 106.
Typically, the piezoelectric material is aluminum nitride (AlN) although zinc oxide (ZnO) or lead zirconium titanate (PZT) are also used. The bottom electrode 102 and top electrode 106 may be a metal such as molybdenum, titanium-tungsten alloy, titanium nitride, tantalum nitride, or other similar metallic material.